This invention concerns a magnetic stroke sensor installed in a fluid-power cylinder which detects the stroke position of the piston rod by reading a magnetic scale embodied in it.
Fluid-power cylinders already exist wherein a piston rod is driven back and forth by means of the pressure of a fluid supplied to the cylinder, and a stroke sensor is fitted in order to detect the position of displacement of the rod. This system may take the form of, for example, a magnetic sensor consisting of a magnetic resistance element in the cylinder which reads a moving magnetic scale embedded at a given pitch on the lateral surface of the rod.
The magnetic sensor is seated in a fixing hole in the cylinder wall, with the magnetic detection surface facing the magnetic scale at a very small distance away. When the piston rod moves with respect to the cylinder, the magnetic flux changes every time the magnetic element forming the scale passes in front of the sensor, and the sensor outputs an alternating waveform of which one cycle corresponds to one passage of the magnetic element. These wave cycles can then be converted to pulses by a comparator, of which the displacement of the piston rod can be found by counting.
If however the distance between the magnetic scale and magnetic detection surface of the sensor facing it changes, the amplitude of the sensor output or its median value will change. In order to obtain a stable output, therefore, it is necessary to control the gap between the sensor surface and the scale very accurately.
Normally, when the magnetic sensor is fitted to the cylinder, a fixing hole is formed in the cylinder wall by a drill or other tool, and the sensor is positioned by bringing a stepped area previously shaped on its rear end into contact with a stepped area formed inside the hole.
The distance between the sensor and the scale is then directly affected by the depth of the stepped area introduced by drilling. In comparison to controlling the cylinder wall thickness and the dimensions of the magnetic sensor, however, it is not so easy to control the depth of the stepped area accurately. This leads either to increased installation cost of the stroke sensor, or to a decrease in its precision of detection and reliability.